EDDY CURRENT PROBE FOR NON-DESTRUCTIVE TESTING WITHIN HOLES IN WORKPIECES OR WITHIN PIPES

Abstract: An eddy current sensor for monitoring the metallization of apertures and narrow-gauge pipes comprises an axisymmetric body (1) elongated along the axis of symmetry, on the inner surface of which is mounted a coil in the form of a helical winding wound along the axis. The body (1) is hollow, the ratio of the outer cross section D to the inner cross section d being such that 1.4 < D/d < 1.6; this, together with the fact that the coil (3) is mounted on the inner surface of the body (1), gives the coil greater mechanical strength and helps to protect it from damage without increasing the cross-sectional dimensions of the sensor. The internal space of the body (1) can be filled with a magnetoconductive material. It is expedient to provide inside the casing a central shaft (8) with a tapering plug at the end (7). The sensor is modified if there is a holder (6). The process for producing the sensor involves forming the coil on a dielectric elastic core (2) and producing the body (1) in its flat developed form; the casing is given its desired final shape after the core (2) with the coil (3) has been mounted on the casing when still flat.

Description: Field of the invention The present invention relates to the non-destructive testing of workpiece properties and, in particular, to eddy-current probes for the non-destructive testing within pipes and holes in workpieces of a small diameter such as testing the coating in the through-holes of printed circuit boards. Normally holes of printed circuit boards vary in diameter from 0.5 to 2.0 mm, whereas thickness of coating in through-holes varies from 15 to 50 microns. The material of electroplating is usually copper. There is not much prior art for testing within apertures of such small dimensions. Known in the prior art is an eddy-current probe for non-destructive testing of electrically conductive plating in holes of printed circuit boards, comprising an oblong cylindrical core with excitation and measuring coils enveloping said core, whose wires are stretched along the axis of said core (US Patent No. 4,072,895). The known method for manufacturing said probe consists of winding an insulated wire on the cylindrical body as its coil (see, for example: Reference Manual Non-destructive Testing, edited by R. McMaster, tr. from English, part 2, Moscow, Energy Publishers, 1965). In the prior art the conductors of the coils envelop the cylindrical core and intersect at the butts or ends thereof. Winding such conductors is a delicate job due to the minute dimensions of the core (less than 1 mm). With manual winding performed with the aid of a microscope electrical characteristics of probes vary considerably. Moreover, conductors of the probe coils may be easily damaged when the probe is inserted into a hole to be tested. Also known in the prior art is an eddy-current probe which is not so easily damaged (see, for example, German Patent DE-A-3,050,497). This probe is intended for non-destructive testing of metal coatings in pipes and holes in workpieces and comprises a carrying cylindrical core and an enveloping coil whose wires are stretched along the axis of the core. All the wires are secured on in the lateral surface of the core. Said coil carries a special coating to be protected from mechanical damage. Moreover, said patent describes the method of manufacturing such probes, including manufacturing of the flat elastic substrate made of dielectric material with the dimensions chosen according to the core size, forming the coil and securing of the formed coil on the surface of the core. The coil forming may be made through, for example, photo-lithography or laser processing of the pre-metallized surface. This method gives some advantages: small dimensions, technologically-based design, high sensitivity, and a probe output which is invariant to changes in the radial and axial placement of the probe with respect to the axis of the hole under test. But during the application this probe has insufficient mechanical stability and may suffer from a short life time for the coil's protective coating. The low durability of this probe is the result of the high core length-to-diameter ratio. Since the probe's outside diameter is defined according to the holes to be tested, the protective coating's enlargement results in the additional reducing of the carrying core's diameter, and so, diminishes the mechanical durability of the probe. This problem becomes acute during the testing of coatings in through-holes of printed circuit boards, when there is presence of both minute core diameter (down to 0.5 mm) and multiple mechanical influence upon the probe. US-A-3,988,665 discloses an eddy-currrent probe having a cylindrical metallic body encircling a hole. The body is encircled by a flexible base sheet with conductors printed on it to get an encircling coil. The probe is intended to test workpieces passing through the hole in the body and cannot be used for nondestructive testing of holes in workpieces, i. e. of through hole coating in printed circuit boards. Summary of the invention. The object of the invention is the solution of the problem of increasing the probe's mechanical stability and resistance to damage without enlarging of the probe's cross-sectional dimensions. According to the invention, there is provided an eddy-current probe for non-destructive testing with the features of claim 1 and with the features of claim 7. Increasing the mechanical durability of the probe is provided through use of a hollow body with a selected ratio of external and internal diameters. This ratio D/d is defined from the transverse dimensions of the external diameter D and internal diameter d and is preferably in a range of 1.4 to 1.6. The coils are secured to an internal surface of the probe's body. The cross sectional area of the probe's body with a ratio D/d = 1.4 is approximately equal to the cross sectional area of the solid body of transverse dimension d. However, due to the tube-like shape said body is more mechanically stable than the solid one having the equal cross sectional area. Since the coils are secured on the internal surface of said body they are less likely to be damaged when the probe is used. This arrangement eliminates the necessity for protective coating of coils and, therefore, the mechanical stability of the probe is provided through increasing the thickness of the body wall but not of the external dimensions of the body. A significant increase in body wall thickness will tend to reduce the probe's sensitivity. According to the findings of the applicant's research a metal probe's body with a ratio D/d up to 1.6 does not influence the probe's sensitivity significantly. However, any further decrease of the dimension d creates a problem of coil forming, due to the minute size of the probe. In the structure of the present invention, bonding pads with holes in the body may be fabricated for channelling wire leads from the internal out to the external surface of the core. The internal cavity of the body may be filled with a magnetically conductive material. The probe's body may have a central core ended with a narrowing plug and mounted inside the internal cavity of the body. The other end of the central core may be spirally formed and fastened in the holder with a hardening adhesive. While a body holder is in use, it is convenient to have the following structure of the probe: a portion of said body with coils extends over one face of the holder, then the body goes through the holder to the other face where the portion of the body coming out of the holder is cut into lobes. Said lobes are bent to make the body be fastened easily to the holder. According to the invention the coils are formed on a planar dielectric elastic substrate and then the substrate is secured to the planar flat reaming of the body. A reaming is defined as a rectangular planar part which one obtains by cutting a cylinder in a longitudinal direction and unrolling it. Then the substrate carrying the coils and the body are rolled into the cylindrical shape of the probe. Brief description of the drawings The following figures show preferred embodiments of the invention which are illustrated in the following detailed description. Fig. 1 illustrates a longitudinal section of the probe; Fig. 2 is the same with the central core inserted; Figs. 3 and 4 are the same with the body coming out of the other face of the holder; and Fig. 5 illustrates the stage of the probe manufacturing before the body is formed. Detailed description of the preferrerd embodiment According to the above, one of the main problems to be solved is the testing of the coating in the through-holes of printed circuit boards, so this particular case is under consideration as an example. The eddy-current probe comprises a body 1 which is formed as a hollow tube with D for an external diameter defined according to the diameter of holes to be tested (e.g. D=0.8 mm). The internal diameter d is defined from the ratio 1.4<D/d<1.6. If D=0.8 mm, dimension of d may vary from 0.5 to 0.57 mm. For smaller values of diameter D the value of diameter d should be chosen closer to the upper limit, and the d values, close to the lower limit, should correspond to the bigger values of D. The tests on bending show the probe fabricated in accordance with the above recommended D/d ratio is more reliable and mechanically stable than the prototype one for the holes of the same diameters in printed circuit boards. The probe sensitivity is acceptable and not influenced by the body made of steel. On the internal surface of the body 1 there is a dielectric elastic substrate 2 upon which are formed spiraled coils 3. The coils 3 are extended along the axis of the body 1. The substrate 2 and the coils 3 correspondingly may be formed in one or several layers and consist of a few sections placed symmetrically. Bonding pads 5 are formed on the substrate 2 at the same time with the coils 3 or connected to the wire leads 4 according to Fig. 1 after the coils are arranged. The body 1 is fastened in the holder 6. The butt of the holder 6 faced to the portion of the body 1 with coils 3 is of cone shape to provide the probe to be easily centered in a hole to be tested. The end of the body 1 is closed with a plug having a tip which has a hemispherical shape or a rounded cone shape. The internal cavity of the body may be filled with any magnetically conductive material, f.e. ferromagnetic elements distributed in a fast-hardening compound. The core 8 closed with cone plug 7 may be mounted inside the body 1 with the coils 3 (see Fig. 2). In this case the portion of the core 8 inserted into the holder may be formed as a rolled up cylinder (a spiral in cross-section) which is easily formed with cores of a small diameter. The spiral part of the core is fastened with the hardening compound to provide the tight mounting of the body 1 in the holder 6. In a preferred embodiment the external wire leads 4 connected to the bonding pads 5 may be brought out to the external surface of the body 1 (see Fig. 2). In this case special holes 9 are made in the body 1 close to the location of the bonding pads 5. The holes 9 may be metallized and electrically connected to the pads 5. While the body 1 is made of the electrically conductive material, the dielectric layer is situated between the surface of the body and the metallizing layer with the proper thickness to provide the reliable insulation. The input/output contacts of the probe are usually placed on the butt of the holder. The preferred embodiment for the contacts is presented in Fig. 3. The body 1 comes out of the holder 6. The out-coming portion of the body 1 is cut to form lobes 11. To provide the mechanical stability the body 1 is made of steel and the dielectrical elastic substrate 2 is situated on the out-coming portion of the body 1, too, to provide the proper insulation of the wire leads 4 from the body 1. The lobes 11 are bent and fastened on the face of the holder 6. The lobes 11 carrying the wire leads 4 are located close to the input/output contacts 10. The contacts 10 may be embedded into the face of the holder 6. The probe proposed is manufactured as follows: a flat reaming is made in accordance with the defined dimensions of the body 1. The height of the reaming equals the height of the body 1 and the width of the reaming equals the perimeter of the body 1. Apart from this the coils 3 are formed on the dielectric elastic substrate 2 by one of the known methods, e.g. thin-film evaporation. In case of multilayer coils 3 conductors of the first layer are plated with a dielectric layer, then the second layer conductors are performed and so on. The inter-layer connections are made through the predrilled holes with metal coating inside. The elastic substrate 2 carrying the coils 3 is put upon the flat reaming or rectangular planar part of the body 1 as shown in Fig. 5. Then the body 1 of the defined diameter is formed from this "sandwich". It is possible to form the coils 3 directly on the surface of the flat reaming of the body 1 by photolithography or laser processing. In this case the surface of the reaming is pre-metallized. If the body 1 is made of steel foil the reaming surface is coated with dielectric before metallizing. After the coils are formed the flat reaming is formed into a tube of a defined diameter. The wire leads 4 may be formed at the same time with the coils 3 or connected to the bonding pads before the tube body is formed or connected from outside when the tube body 1 is already formed. The formed sensitive portion of the body is fastened in the holder 6. The external contacts are mounted. The eddy-current probe operates in the following manner: The coils 3 of the probe are connected to, for example, the resonant circuit of the oscillator or to the bridge circuit which has one branch connected to the oscillator and the other to the measuring device or set (such test sets are well-known in the prior art). The probe is inserted into a hole under test in coaxial alignment; insertion is aided by the use of the cone plug tip. An oscillating signal from the test set provides an alternating current in the coil 3. This creates an alternating magnetic field around the coil which excites an eddy current in the metal plating (or coating) within the hole under test, i. e. within the through hole of a printed circuit board. The test set oscillator is adjustable for frequency and a frequency is selected which will be most revealing of defects for a given hole under test. For a given plating thickness and a given set of axial or peripheral cracks within the plating the resulting current in the plating will vary. Thus, the magnetic field in the coil and the EMF excited as a result of the initiation of the measurement can be seen to vary with that eddy current. The probe of the present invention performs these functions while providing greater mechanical stability and greater protection from damage to the measuring coil. Industrial application The invention may be applied in all industries connected to the manufacturing and application of the printed circuit boards for non-destructive testing the coating in the through-holes, for measuring the geometrical dimensions of the holes in metal sheets, profiles and so on, for measuring the characteristics of the electro conductive material of the coating in the holes and pipes (especially electrical conductivity, magnetical properties), for quality control of the surfaces of the holes in workpieces and pipes, e.g. routines of the thermal and thermochemical processing, to discover the overheated areas while machining and so on.

Claims: An eddy-current probe for nondestructive testing within holes in work pieces or within pipes, comprising a hollow cylindrical metallic body (1) having an inner surface with an inner diameter d, an outer surface with an outer diameter D and a central axis, an elastic dielectric substrate (2) having an inner surface and an outer surface and conductor coils (3), fabricated onto said substrate inner surface, having its conductors (4) arranged parallel to the body central axis, and wherein said substrate (2) outer surface is juxtaposed with said metallic body (1) inner surface and the ratio D/d is in the range of 1.4 to 1.6. An eddy-current probe as claimed in claim 1, wherein said coils (3) include bonding pad terminations (5) which are connected to wire leads (4) through holes (9) made in said body (1) close to the locations of the bonding pads (5) of the respective coils (3). An eddy-current probe as claimed in claim 1 or 2, further comprising magnetically conductive material disposed within said hollow body (1). An eddy-current probe as claimed in one of the claims 1 to 3, further comprising a central core (8) mounted within said hollow body (1) said core (8) having a first end and a second end, said second end terminating in a rounded conical tip (7). An eddy-current probe as claimed in claim 4, wherein said central core (8) within said body (1) is affixed to a holder (6). An eddy-current probe as claimed in claim 1, further comprising a holder (6) having a first top side, a second bottom side, and a cylindrical aperture which places the two sides in communication and is adapted to accept and support said metallic cylindrical body (1); wherein a portion of the body (1) is situated on the bottom side of the holder (6) and the body (1) goes through and comes out on the top side of the holder (6) said body (1) further including lobes (11) which are bent and fastened on the top side of the holder (6). An eddy-current probe for nondestructive testing within holes in work pieces or within pipes, comprising a hollow cylindrical metallic body (1) having an inner surface with an inner diameter d, an outer surface with an outer diameter D and a central axis, an elastic dielectric substrate (2) having an inner surface and an outer surface, conductor coils (3), fabricated onto said substrate (2) inner surface, having a conductor (4) arranged parallel to the body (1) central axis, and wherein said substrate (2) outer surface is juxtaposed with said metallic body (1) inner surface and wherein a reaming of said metallic body (1) and said flexible substrate (2) are rolled together to form a cylindrical probe structure having the ratio D/d in the range of 1.4 to 1.6.

Inventors: GORSKAYA LJUDMILA EVGENIEVNA, ULITIN YURY MIKHAILOVICH, GORSKAYA, LJUDMILA EVGENIEVNA, ULITIN, YURY MIKHAILOVICH
Applicants: INTRON PLUS LTD, INTRON PLUS LTD.
Application year: 1994
Main classification: G01N27
IPC codes: G01N27